btr_pcur_get_rel_pos(): Add a const qualifier.

btr_pcur_get_btr_cur(), btr_pcur_get_page_cur(): btr_cur_get_page_cur():
Define as const-preserving macros.

btr_pcur_is_on_user_rec(), btr_pcur_is_after_last_on_page(),
btr_pcur_is_before_first_on_page(): Remove the unused parameter mtr.
Add a const qualifier.

btr_pcur_move_to_next_on_page(), btr_pcur_move_to_prev_on_page():
Remove the unused parameter mtr.

page_cur_search(): Add const qualifiers.

page_cur_get_page(), page_cur_is_before_first(), page_cur_is_after_last():
Add debug assertions.